1. Field of the Invention
The present invention relates to a fixing unit to fix a toner image on a recording medium by heating and pressing the recording medium, and an image forming apparatus provided with the fixing unit.
2. Description of Related Art
An image forming apparatus of an electrophotographic system such as a copier, a printer, and a facsimile machine, as well as a multifunction peripheral having such functions is available. In the fixing unit of this image forming apparatus, a heat roller system using a heat roller serving as a fixing member is widely used. For example, a fixing unit of this heat roller system is used in machines ranging from low-speed machines to high-speed machines and further from monochrome machines to color machines. The heat roller is also referred to as a fixing roller, and the heat roller system is also referred to as a heat-fixing roller system.
A fixing unit of such a heat roller system is provided with a fixing roller which is kept at a predetermined temperature and a pressure roller as a pressure member having an elastic layer. A recording medium on which an unfixed toner image has been formed is conveyed while being nipped by a fixing nip portion formed by the fixing roller and the pressure roller. Then, the fixing unit of the heat roller system heats and presses the thus-conveyed recording medium.
Over recent years, instead of the above fixing roller, an endless fixing belt stretched by at least two rollers is being used as a fixing member. A belt fixing unit having such a fixing belt is provided with a pressure roller, serving as a pressure member, to press the fixing belt. A recording medium on which an unfixed toner image has been formed is heated and pressed by a fixing nip portion formed by the fixing belt having been heated by at least one of the rollers stretching the fixing belt and the pressure roller.
As a recording medium is conveyed to the fixing unit in this manner, the surface of the fixing member is damaged by the recording medium and then gradually roughened. The damage is mainly caused by burrs of both edges of the recording medium produced during cutting of the recording medium. Namely, when a recording medium is conveyed to the fixing unit, buns in both edges of the recording medium in the longitudinal direction of the fixing roller roughen the surface of the fixing member. Especially when recording media of the same size are continuously conveyed to the fixing unit, locations of the fixing member corresponding to both edges of the recording media, namely, locations of the fixing member through which the above burrs are passed are damaged, resulting in the state of being roughened. Such roughness of the surface of the fixing member produces gloss non-uniformity in an image to be fixed on a recording medium, resulting in occurrence of image degradation.
Therefore, for example, as a related technology, Unexamined Japanese Patent Application Publication (hereinafter referred to as JP-A) No. 2-266383 describes a fixing unit having a web to clean a fixing roller. A metal is attached to this web at a rate of 0.5 through 1.5 g/m2. The web grinds the fixing roller serving as a fixing member, whereby the surface of the fixing roller is uniformed. Thereby, the smoothness (hereinafter referred to as surface property) of the surface of the fixing roller is recovered. However, the inventors of the present invention found a problem in which grinding of the fixing roller shortened the lifetime of the fixing roller.
Further, for example, as another related technology, JP-A No. 2008-40363 describes an image heating unit (namely, a fixing unit) having a heating rotating body (namely, a fixing roller) to heat an image on a recording medium at a nip portion and a friction sliding section to recover the surface property of the heating rotating body.
It is disclosed therein that this friction sliding section has a friction sliding layer formed by adhesion of dense abrasive grains, and the heating rotating body is subjected to scratches by the friction sliding section, whereby the surface property of the heating rotating body is recovered, namely, the friction sliding section produces a large number of fine scratches on the surface of the fixing roller, whereby the surface property of the fixing roller is recovered.
However, repetitive recovery operations of the surface property of the fixing roller cause contamination of the friction sliding section. Namely, the inventors of the present invention found that since the friction sliding section was contaminated, the roughness of the friction sliding section was unable to be maintained, whereby the recovery function of the friction sliding section was decreased.